Monolithic voltage controlled oscillators (VCOs) are widely used elements in typical communication systems. VCOs are typically used in wireless transmitter/receiver (T/R) building blocks, such as a Phase Locked Loop (PLL) type frequency synthesizer. Although many applications require a single fixed frequency, the VCO must still be capable of providing adequate tuning range to cover shifts in center frequency which result from normal parametric variations found in any semiconductor process. In addition to serving as tunable oscillator within a PLL, many T/R applications require both In-phase (I) and Quadrature (Q) phase Local Oscillator (LO) signals, for In-phase and Quadrature (I-Q) conversion of voltage signals. Since the ring-type oscillator oscillates with a total additive phase shift around the loop of 360 degrees, I-Q signals can easily be produced using an even number of delay elements in the loop, and closing the feedback loop with a wired inversion, such as with differential signals which provides 180 degrees of phase shift. Thus, with two delay elements, each element provides 90 degrees of phase shift at the frequency of oscillation.
Typical prior art monolithic ring type VCO, such as shown in U.S. Pat. No. 4,884,041, uses a linear combination or interpolation technique consisting of a plurality of simple logic gate inverters and a linear combiner circuit combined to achieve variable delay. According to the prior art invention, the minimum number of functional components for a VCO with quadrature outputs are two logic gates and two combiner circuits. However, it is desirable to provide a VCO that provides over an octave of tuning range using fewer components and less power to thereby lower device cost, whereas the invention herein achieves the same function with the equivalent complexity of two combiner circuits. The elimination of two logic gates in a bipolar implementation saves 14 transistors and 10 resistors.